Preheat the oven to 180 degrees C. Line a high sided, 18cm x 25cm high sided metal baking tin with greaseproof baking paper
To make the blackberry caramel, put the blackberries in a non-stick, non-reactive pan and heat over a medium to high heat with two tablespoons of water and a pinch of salt. Cook for a few minutes, until the blackberries change colour and start to release their liquid and become squishy. Remove to a bowl and squash with the back of a fork. Wipe out the pan and melt the sugar slowly. Add the butter and crème fraiche and cook for 3-5 minutes, until you have a smooth caramel sauce. Add in the blackberries and their juice and cook, stirring into the caramel. You should have a lovely, slightly purple fruity caramel. Season with salt to balance and bring out the flavours
To make the blondies, in a medium bowl, stir melted butter and brown sugar until smooth. Add the egg, vanilla, salt and stir vigorously until smooth. Add the flour and half the almonds and stir until no streaks remain. Spread half the batter across the baking tin, top with the blackberry caramel, spreading it out as evenly as you can. Top with the rest of the batter and spread out over the caramel, using a toothpick to marble. Press in the remaining blackberries and scatter over the flaked almonds. Bake for 20 to 25 minutes until a toothpick inserted into the middle comes out relatively clean. Transfer quickly and efficiently onto a wire rack to cool, then cut into squares.